In a case that a potential higher than a basic potential of unit battery (cell) is necessary, such as a hybrid vehicle using a lithium ion cell as a power source, it is common to use a plurality of unit batteries which is connected in series. However, even though the batteries are produced with the same structure via a typical production method using the same anode, cathode and electrolyte material, a difference in charging or discharging (and self discharging) characteristics exists between each of the batteries connected in series.
Therefore, a potential difference can exist between the unit batteries when using the batteries connected in series.
Even if one battery of the unit batteries connected in series is perfectly discharged regardless of a potential of other battery, a total voltage (total voltage of batteries connected in series) becomes zero so that the battery is need to be recharged. Upon recharging the battery, since the potentials of the batteries are different from one another, there is a problem of over-charging in that the battery may reach a prescribed voltage in advance, as well as a problem of charge inefficiency in that some batteries may not reach the prescribed voltage even in an existence of an over-charging.
Further, if the number of charging/discharging times becomes large, degradation is caused in materials composing the battery and thus a property of the battery is varied, and as a result, such degradation situation is responsible for further increasing a difference in individual cells.
In order to address such problems, there are proposed various charge-equalization apparatuses which can accomplish charge equalization of the batteries connected in series.
As an example, Korean patent Laid-Open No. 2007-0031406 is directed to an apparatus and method for equalizing the charges of series-connected individual cells of an energy storage using a DC/DC converter to which the energy is supplied from the energy storage or other energy source, and more specifically, the DC/DC converter charges an intermediate circuit capacitor, and a voltage of the intermediate circuit capacitor is reversed by the DC/AC converter so that the alternating voltage is converted to a pulsed direct current by a rectifier through AC bus lines and combined transformer and the cell having lowest voltage is charged with the pulsed direct current.
Korean Patent Laid-Open 2007-0006762 is directed to an apparatus for equalizing charge of series-connected capacitors of 2-step capacitor (DLC) in which individual transformers Tr1 to Trn assigned to each of capacitors C1 to Cn respectively and a voltage comparator are provided and a secondary winding of the individual transformer is connected via the individual diodes D1 to Dn to an anode terminal of the capacitor and directly connected to a cathode terminal of the capacitor.
European Patent Registration No. EP 0432639 is directed to a charge equalizing apparatus which equalizes charge between low-charged battery and the remaining batteries if a plurality of batteries is connected in series by providing charging circuit including a rectangular-functional generator and a comparing circuit and diode, transformer and contact breaker, for each individual battery of a battery stack.
U.S. Pat. No. 5,659,237 is directed to a charge equalizing apparatus which equalizes charge of at least 2 series-connected energy storage cell, comprising a transformer having a primary winding and a secondary winding (herein, the secondary winding is connected to one of an energy storage cells), a forward converter for inducing the charging current in each of the secondary windings by applying the equalizing voltage signal to the primary winding of the transformer, and a means for increasing a magnitude of the voltage signal until the energy storage cells are equalized while charging lower cells.
Specifically, the prior technology can be represented as FIG. 1, in which batteries B1 to BN are connected in series in a center portion and DC/DC converters 110_1 to 110_N are connected to every two adjoining batteries. The DC/DC converter is implemented in such a way to discharge the energy from top battery and charge the energy to bottom battery of the two adjoining batteries. In a case of undermost-located battery B1, the DC/DC converter is implemented such that the discharged energy is charged to an uppermost-located battery BN of the series-connected battery string. The charge equalizing apparatus in FIG. 1 allows the charge equalization to be accomplished by moving charge from the top cell to the bottom cell of two adjoining cells when the charge imbalance is caused.
FIG. 2 shows an example of the charge equalizing apparatus having an automatic PWM generating means 210 for the nth cell. As shown in FIG. 2, a forward converter 220 is provided between two batteries Bn+1, Bn connected in series, and an input of the forward converter is connected to the top cell and an output of the forward converter is connected to the bottom cell respectively. An anode terminal and a cathode terminal of a comparator 212 are connected to an average potential of the batteries and a potential of the nth battery Bn respectively. An output of the comparator 212 is used to drive a PWM generator 211 and the PWM signal generated by the PWM generator 211 is used to drive the forward converter 220. In the charge equalizing apparatus having an automatic PWM generating means 210 shown in FIG. 2, the PWM signal is generated when the potential of the top cell Bn+1 is higher than that of the bottom cell Bn of the two batteries connected in series, and the PWM signal operates the forward converter 220 and as a result, the charge equalization is accomplished in such a way that the charge is moved from the over-charged top cell to relatively low-charged bottom cell.
In the prior automatic charge equalizing apparatus, the charge equalization is accomplished in such a way that the charge is moved from the over-charged battery to relatively low-charged battery by comparing potentials of the two adjoining batteries. Herein, the automatic charge equalizing apparatus determines the potential of two adjoining batteries and initiates the charge equalization operation if the potential of the top battery is higher than that of the bottom battery. Therefore, the prior technology has limitations in that the charge is discharged from the cell having a low potential if two cells having relatively low potential of the series-connected battery string are adjoining. In other words, even though some adjoining cells are low-charged at the same time, since the charge equalizing apparatus is operated by determining the potential between the two adjoining cells, the energy can be discharged from the relatively low-charged battery cell. In order to address such problem, the charge equalizing apparatus can be structured such that the energy is discharged only if the potential of the current cell is higher by comparing the average potential of total battery cells and the average potential of the current cell of interest, which results from complexity of the charge equalizing apparatus.